Refractive disorders of the eye include myopia (bad far sight), hyperopia (bad near sight), astigmatism, and presbyopia. In myopia, the eye can focus at shorter distances (e.g., less than 30 cm to less than infinity), but cannot focus at infinity. In hyperopia, the eye can focus at or past infinity, but cannot focus at some shorter distances (e.g., at 30 cm). When the crystalline lens is completely relaxed: (a) a perfect eye focuses parallel rays to a single point on the retina; (b) an eye with myopia focuses parallel rays to a point before the retina; and (c) an eye with hyperopia focuses parallel rays to a point after the retina.
Astigmatism is a refractive condition caused by a toric curvature in the cornea or in the crystalline lens. In this case the eye has different curvatures along two perpendicular meridians; thus, it cannot sharply focus an image on the retina. Put differently, an astigmatic eye refracts light in a radially asymmetric manner.
Accommodation is the adjustment of the shape of the eye's crystalline lens, in order to focus on an object. During accommodation, the shape of the lens changes from more planar (when the eye is focusing at a far scene) to more spherical (when focusing on a near object).
Presbyopia is a reduction in the eye's ability to perform accommodation experienced by most individuals starting at the age of 40. As such, it is considered a natural stage of the eye's aging process.
Refractive correction for myopia, hyperopia, and astigmatism can be achieved with the use of corrective lenses. The refractive power of a lens is expressed in diopters, a unit defined as the reciprocal of the lens' focal length expressed in meters. A diverging lens (negative diopters) may be used to correct myopia. A converging lens (positive diopters) may be used to correct hyperopia.
Refractive eye disorders are the 2nd leading cause of blindness worldwide. [World Health Organization, Visual impairment and blindness, Factsheet, 2010] Over 153 million people around the world suffer from uncorrected refractive vision problems and 87% of them—133.11 million people—reside in developing nations. [B. Holden, Uncorrected refractive error: the major and most easily avoidable cause of vision loss, Community Eye Health, Vol. 20(63), pp. 37-39, 2007] Many of these sufferers do not have access to appropriate diagnostic and treatment options due to cost—they live on less than a dollar a day—and due to an insufficient number of optometrists. Lots of them do not even know that the vision difficulties or headaches are due to refractive errors. For many children, hyperopia is not diagnosed since they can see objects clearly. However, because they are putting extra effort to focus at infinity, over time it added stress and headache. All these problems lead to a tremendous loss in productivity in the developing countries, with estimated losses ranging from USD 88.74 to USD 133 billion.
It would be highly desirable to have a low cost, accurate tool for assessing refractive disorders, particularly for use in some developing countries, where optometrists are in short supply or patients cannot afford an optometrist. It would be very helpful if such a tool could be used for screening and diagnosis, and to determine the spherical power, cylindrical power and cylindrical axis of the eyeglass prescription needed to correct the refractive disorder.